os/xdg: Add tests for os::font_paths() interactions w/ env vars

os/BUILD

@@ -109,3 +109,15 @@ cc_test(
         "//etest",
     ],
 )
+
+cc_test(
+    name = "xdg_linux_test",
+    size = "small",
+    srcs = ["xdg_linux_test.cpp"],
+    copts = HASTUR_COPTS,
+    target_compatible_with = ["@platforms//os:linux"],
+    deps = [
+        ":xdg",
+        "//etest",
+    ],
+)

os/xdg_linux_test.cpp

@@ -0,0 +1,62 @@
+// SPDX-FileCopyrightText: 2021-2023 Robin Lindén <[email protected]>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+// https://man7.org/linux/man-pages/man3/setenv.3.html
+#ifdef _POSIX_C_SOURCE
+#undef _POSIX_C_SOURCE
+#endif
+// NOLINTNEXTLINE: The only option is to mess with this reserved identifier.
+#define _POSIX_C_SOURCE 200112L
+
+#include "os/xdg.h"
+
+#include "etest/etest2.h"
+
+// This is the header POSIX says we need to include.
+// NOLINTNEXTLINE(modernize-deprecated-headers)
+#include <stdlib.h>
+
+// NOLINTBEGIN(concurrency-mt-unsafe): No threads here.
+
+int main() {
+    // Ensure that the system's environment doesn't affect the test result.
+    unsetenv("HOME");
+    unsetenv("XDG_DATA_HOME");
+
+    etest::Suite s{"os::xdg/linux"};
+
+    auto const font_paths_without_env_vars = os::font_paths();
+
+    s.add_test("HOME", [&](etest::IActions &a) {
+        static constexpr auto kHome = "/home";
+        setenv("HOME", kHome, false);
+
+        a.expect(std::ranges::find_if(font_paths_without_env_vars, [](auto const &path) {
+            return path.contains(kHome);
+        }) == font_paths_without_env_vars.end());
+
+        auto font_paths = os::font_paths();
+        a.expect(std::ranges::find_if(font_paths, [](auto const &path) { return path.contains(kHome); })
+                != font_paths.end());
+        unsetenv("HOME");
+    });
+
+    s.add_test("XDG_DATA_HOME", [&](etest::IActions &a) {
+        static constexpr auto kXdgDataHome = "/xdg_data_home";
+        setenv("XDG_DATA_HOME", kXdgDataHome, false);
+
+        a.expect(std::ranges::find_if(font_paths_without_env_vars, [](auto const &path) {
+            return path.contains(kXdgDataHome);
+        }) == font_paths_without_env_vars.end());
+
+        auto font_paths = os::font_paths();
+        a.expect(std::ranges::find_if(font_paths, [](auto const &path) { return path.contains(kXdgDataHome); })
+                != font_paths.end());
+        unsetenv("XDG_DATA_HOME");
+    });
+
+    return s.run();
+}
+
+// NOLINTEND(concurrency-mt-unsafe)